This invention relates to composite materials and more particularly to tough, impact resistant fiber-reinforced composites. Still more particularly, this invention relates to methods for toughening fiber-reinforced composites and to particles useful in toughening such composites.
Fiber-reinforced composites are high strength, high modulus materials which are finding wide acceptance for use in sporting goods and in producing consumer items such as appliances. Such materials are also finding increased acceptability for use as structural components in automotive applications, as components of buildings and in aircraft. Typically, the composites used in structural applications comprise structural fibers in the form of continuous filaments or woven cloth embedded in a thermosetting or thermoplastic matrix. Such composites may exhibit considerable strength and stiffness, and the potential for obtaining significant weight savings makes them highly attractive for use as a metal replacement. However, acceptance for many structural applications has been limited by the fact that many of the composite materials presently available are brittle. The inability of such composites to withstand impact while retaining useful tensile and compression strengths has been a serious problem for many years. Compensating for the low impact resistance of such materials may ordinarily be accomplished by increasing the amount of material employed. This approach increases costs, reduces the weight savings that might otherwise be realized and may make them unacceptable for many uses.
The composites industry has long been involved in finding ways to overcome these deficiencies. Considerable effort has been expended over the past two decades directed toward the development of composites with improved fracture toughness. Inasmuch as most of the commonly employed matrix resins, as well as many of the reinforcing fibers, are generally brittle, much of that effort has gone into a search for components having better toughness characteristics. As a consequence, the search for toughened matrix resins has become the subject of numerous recent patents and publications.
For decades, the plastics industry has used rubber modifiers to toughen rigid, frequently brittle thermoplastic and thermoset engineering resins. Most often the rubber is dispersed in the form of particles throughout the rigid resin. Various means for altering the interaction between the rubber particle and the rigid phase to improve the effectiveness of the rubber component have also been explored. For example, the rubber components have been modified by grafting to change compatibility with the rigid phase, and adding reactive functional groups to the rubber to promote bonding to the rigid phase has also been shown to be effective. Other approaches have included the combining of dissimilar resins, forming blends and alloys with improved properties.
The methods used for toughening engineering resins have been adapted for the toughening of the matrix resins commonly used in composite structures, as shown for example by Diamant and Moulton in "Development of Resin for Damage Tolerant Composites--A Systematic Approach", 29th National SAMPE Symposium, Apr. 3-5, 1984. The forming of alloys and blends by adding a more ductile thermoplastic such as a polysulfone to an epoxy resin formulation has also been shown to improve the ductility of the epoxy resin and provide enhanced toughness, according to British patent 1,306,231, published Feb. 7, 1973. More recently, combinations of an epoxy resin with terminally functional thermoplastics were shown to exhibit enhanced toughness. See U.S. Pat. No. 4,498,948. Still more recently, curable combinations of epoxy resins and thermoplastics with reactive terminal functionality were also said to improve the toughness of specifically formulated matrix resins, provided that the neat resin after curing exhibits a specific phase-separated morphology having a cross-linked glassy phase dispersed within a glassy continuous phase. See U.S. Pat. No. 4,656,208. Further improvements are said to be achieved by including a reactive rubber component which is said to be contained within the cross-linked dispersed glassy phase. See U.S. Pat. No. 4,680,076. Still more recently, the use of an infusible particle made from a rubber dispersed within the phase-separated cross-linked epoxy resin matrix has been suggested for toughening composites based on such matrix resins. See U.S. Pat. No. 4,783,506.
Although the addition of rubber, thermoplastics and the like generally improves the ductility and impact resistance of neat resins, the effect on the resulting composites is not necessarily beneficial. In many instances the increase in composite toughness may be only marginal, and a reduction in high temperature properties and in resistance to environmental extremes such as exposure to water at elevated temperatures frequently is seen. Composite structures that rely on complex manufacturing methods or on unique resin morphologies that are difficult to reproduce for achieving improvements in toughness may require an impractical degree of control during fabrication, adding to the production costs and often resulting in erratic performance and poor reliability.
An alternative approach to producing toughened composites has been the development of layered composite structures having layers formed of fibers imbedded in a matrix resin alternated with layers formed of a thermoplastic resin, described in Japanese patent application 49-132669, published May 21, 1976. More recently, in U.S. Pat. No. 4,604,319, there were disclosed layered fiber-resin composites having a plurality of fiber-reinforced matrix resin layers inter-leafed with thermoplastic layers adhesively bonded to the reinforced matrix resin layers. Inter-leaf structures are ordinarily produced by impregnating continuous fiber to form prepreg, then laying up the composite by alternating prepreg with sheets of thermoplastic film. The laid-up structure is then subjected to heat and pressure, curing the matrix resin and bonding the layers. The patent also discloses inter-leaf layers which comprise a thermoplastic filled with a reinforcing material such as chopped fibers, solid particles, whiskers and the like.
Although inter-leafed composite structures with improved toughness have been disclosed, there has been some sacrifice in other physical properties, including a reduction in glass transition temperatures together with an increase in creep at high temperatures. Further difficulties with such composites may include a loss in stiffness for many such compositions, adhesive failure that may occur between layers formed of dissimilar resins and property deterioration during use due to poor solvent resistance. In addition, prepregs based on thermoplastic resin generally are lacking in tack, which complicates their fabrication into composites and increases the degree of skill needed to fabricate complex structures. This may in turn result in increased scrap losses and a need for more complex quality control procedures, increasing manufacturing costs in order to achieve an acceptable level of reliability.
The compositions and methods presently available for producing toughened composites thus require further improvement. Composites having improved resistance to impact and particularly those with better compressive strength after impact would be a useful advance in the art, and reliable methods for producing such toughened composites could find rapid acceptance, displacing the more complex and expensive manufacturing processes currently available for these purposes.